The Math Behind Why, When, and by How Much?
Raising and lowering seeding rates is one of the easiest fall management strategies, yet it is often overlooked and undervalued. Before we work through some math, we first have to understand the purpose of adjusting seeding rates. To obtain optimal yield in a wheat crop, we want to see 450-600 good heads per square yard. We refer to this area as the “SWHEAT SPOT”. This ensures we have optimal yield potential while minimizing lodging risk.
Increasing or decreasing seeding rates based on calendar date is one management strategy to ensure we hit the SWHEAT spot. For instance, if the planting date is September 24 and you are located in South Huron sowing 1,600,000 million seeds per acre, you will achieve 23-24 plants per foot of row (37plants/ft²).
It takes roughly 80 Growing Degree Days to begin the germination process and an additional 50 GDD to emerge per inch of planting depth (Pub 811). Using September 24th as a planting date, there were 468 GDD up to December 31st in South Huron. Planting at 1.25” deep, it took roughly 143 GDD for the wheat to emerge. Following emergence, it takes 100 GDD per leaf to fully develop. Using the chart above, that would put the potential head count right in the middle of the SWHEAT spot which is perfect.
On the flip side, if the planting date was delayed until October 15th, there were 180 GDD from planting to the end of December in South Huron. Planting 1.25” would still require 143 GDD for emergence. That leaves us with 37 GDD; this wheat has emerged, but very little growth has occurred since emergence.
This is the exact reason why seeding rates need to increase as we move later into fall. This wheat has not developed any tillers going into winter and if seeding rates were not kept up to compensate for lack of tillering, yield potential will be reduced. If seeding rates were kept up in the 1.8-1.9M seeds/ac range (27-29 seeds per ft) there is still good potential. The best strategy to maintain yield in this situation is to apply a “kickstart” application of nitrogen to encourage tillering and promote early growth.
The rule of thumb is to target 1.6 million seeds/ac at the optimum date for the region you are in (see map below). If you are planting before the optimum date, reduce rates by 100,000 seeds/ac/week. If planting past the optimum planting date, increase by 100,000 seeds/ac/week to a maximum of 2.2 million seeds/ac. If seeding on heavy clay, it is recommended to plant 10-15% more right off the bat. Another factor to consider is seed cost savings from early planting. If you can plant two to three weeks earlier, seed cost savings will be roughly 15-20%.
• Planting Early -> Less seeds/ft of row -> More GDD -> More Tillers = 450-600
• Planting Late -> More seeds/ft of row-> Less GDD-> Less tillers = 450-600
If 150lbs/ac is your standard rate year after year regardless of seed size and calendar date, potential is being lost. If your variety is 12,500 seeds/lb and you are seeding late September in Middlesex County, that equates to 1,875,000 seeds/ac. This is too much seed, extra seed costs were incurred and lodging potential is increased. If the same variety was being planted the end of October in the same region, there may not have enough seed to maximize potential depending on the growing season following planting. The key message is make sure you know what your seed size is and ensure your seeding equipment is calibrated to accurately put down the desired rate based on calendar date.
The potential for your wheat crop is actually determined this spring. Have your “Wheat Bean” selected and get the wheat in on time. We can decrease seeding rates as we plant earlier and yields will increase because the plants can capitalize on more GDD to advance development. The rule of thumb is that every day planting is delayed, 1.1 bu/ac/day is lost. There is some new anecdotal data that suggests the bu/ac/day lost is even more than we think.
Read a past edition of The Wheat News Plus “What is Your Wheat Bean”